1. Field of the Invention
The present invention relates to:                a novel process for the crosslinking of at least one polymer selected from polysaccharides and derivatives thereof;        a process for the preparation of an injectable monophase hydrogel of at least one such polymer; and        the crosslinked polymers and injectable monophase hydrogels respectively obtainable by each of said processes.        
The hydrogels in question, based on said crosslinked polymers, have numerous outlets, especially as filling materials in plastic, cosmetic and dental surgery, in ophthalmology, in orthopedics, etc., as products for preventing tissue adhesions, in general surgery, in urology, etc. Said hydrogels are particularly suitable for repairing vocal cords. The outlets indicated above for products of this type, without implying any limitation, are familiar to those skilled in the art.
The invention is the result of a genuine effort to optimize the operation of crosslinking the polymers in question with a view to obtaining injectable monophase hydrogels that are of particular value in respect of the following compromise: on the one hand mechanical properties and remanence, and on the other hand injectability (with acceptable injection forces and injection needle diameters).
It is pointed out here that the term “injectable” employed in the present text, with reference to both the hydrogels of the prior art and the hydrogels of the invention, denotes manual injectability by means of syringes equipped with conventional needles (having a diameter of between 0.1 and 0.5 mm). Within the framework of the present invention, it is possible in particular to formulate hydrogels that can be injected through hypodermic needles of 30 G½, 27 G½, 26 G½ and 25 G.
2. Discussion of Related Art
According to the prior art, hydrogels, especially injectable hydrogels, have already been prepared from polysaccharides and derivatives thereof—especially hyaluronic acid salts—having a zero, low or high degree of crosslinking.
With reference to the specific problem of injectability, biphase compositions have been proposed whose continuous phase, in particular, is based on such hydrogels. The continuous phase serves as a plasticizer, injection vehicle for a disperse phase. This disperse phase is more or less solid and more or less differentiated from the continuous phase. Thus:                the biphase compositions described in patent application EP-A-0 466 300 consist of two bioabsorbable phases—continuous and disperse—and take the form of slurries. Said two phases are advantageously prepared from fibers of Hylan (natural hyaluronic acid chemically modified in situ in order to facilitate its extraction from the tissues);        the biphase compositions described in patent application WO-A-96 337 51 also have two bioabsorbable phases with a better separation, the disperse phase consisting of insoluble fragments of a highly crosslinked polymer hydrogel (selected from hyaluronic acid and its salts);        the biphase compositions described in patent application WO-A-00 014 28 contain a non-bioabsorbable disperse phase (particles of at least one hydrogel of a (co)polymer obtained by the polymerization and crosslinking of acrylic acid and/or methacrylic acid and/or at least one derivative of said acids) suspended in an aqueous solution of a crosslinked or non-crosslinked polymer selected from proteins, polysaccharides and derivatives thereof.        
These biphase systems are not fully satisfactory insofar as they are associated with justifiable fears of uneven flow during injection and particularly after injection, a more rapid disappearance of the continuous phase (having a zero or low degree of crosslinking) and hence an at least partial loss of the desired effect, especially filling effect.
Monophase hydrogels, developed from the same types of polymers, were therefore also proposed in parallel.
In patent applications WO-A-98 356 39 and WO-A-98 356 40, the product in question is not an injectable hydrogel but a product of solid consistency. Said patent applications in fact describe ocular implants used to temporarily fill a surgically created void. The hydrogel developed in U.S. Pat. No. 4,716,154 is proposed as a substitute for the vitreous body. The polymer in question (sodium hyaluronate) has a very low degree of crosslinking in order to obtain an injectable hydrogel. The monophase hydrogel described in patent application WO-A-02 057 53 is laden with an antiseptic that is effective in protecting it from free radicals after implantation. Patent application WO-A-02 063 50 describes a process capable of generating this type of hydrogel that is very homogeneous throughout.
All these monophase hydrogels were obtained from high-molecular weight polymers crosslinked using an effective and non-excessive amount of at least one crosslinking agent, in an aqueous solvent.
In the light of this prior art, the inventors wished to improve the efficacy of crosslinking of the polymer in question, especially in order to improve the degradation resistance (remanence) of the implanted hydrogel while at the same time preserving the possibility of injecting said hydrogel under acceptable conditions.
To improve the crosslinking efficacy, the inventors initially considered using more crosslinking agent. This approach was quickly discarded on the grounds that it inescapably causes denaturation of the polymer in question and chemical contamination of the crosslinked product obtained.
Said inventors then considered increasing the concentration of polymer in the reaction mixture. In the same way, this second approach had to be discarded, a priori, because of the polymers conventionally used hitherto, namely high-molecular weight polymers. Thus sodium hyaluronate is always used with high molecular weights (Mw≧106 Da, ≈2·106 Da, 3·106 Da) at concentrations close to the maximum concentration, which is about 105-110 mg/g. Using it at a higher concentration is difficult (the viscosity of the reaction mixture becomes too high) and inescapably causes problems of solubility, poor homogeneity, etc.
Concentrating the reaction medium, on the other hand, is found to be possible with low-molecular weight polymers (sodium hyaluronate of molecular weight 300,000 Da, having an intrinsic viscosity of 600 ml/g (those skilled in the art are perfectly familiar with the relationship between these two parameters: molecular weight (M) and intrinsic viscosity (η), which is given by the Mark-Houwink formula: M=kηα, the values of k and α depending on the nature of the polymer in question), can be concentrated from 110 to 200 mg/g). Unfortunately the crosslinked polymer obtained generates an inhomogeneous, injectable biphase hydrogel under these conditions.
In such a context, the inventors surprisingly established that associating low-molecular weight polymer(s) with high-molecular weight polymer(s) affords an excellent compromise, namely the possibility of generating, for a non-excessive degree of crosslinking (equivalent to that of the prior art), an injectable monophase hydrogel which has improved mechanical and remanence properties. This low-molecular weight/high-molecular weight association makes it possible to obtain a hydrogel that more than satisfies the following specifications:                monophase;        better mechanical properties and remanence than the equivalent products of the prior art;        unaffected or even improved injectability that is still possible with conventional injection forces using conventional injection devices.        
The key factor of the crosslinking process of the invention therefore lies in the concentration of the reactants (which is greater than that of the reaction mixtures of the prior art due to the use of low-molecular weight polymer(s)), although the crosslinking of said concentrated reactants is “governed” by the use of high-molecular weight polymer(s), which guarantee the homogeneity of the crosslinked product obtained and then of the hydrogel obtained.